The enhancement of the maneuverability and the stability of a vehicle is an eternal challenge for researchers related to a motion of a vehicle. Currently, the following three methods are proposed from Japan to the world.
(1) Four-Wheel Active Steer System
A patent literature 1 is based a four-wheel active steer system provided with a front-wheel active steer means that applies an auxiliary steering angle to a front wheel, a rear-wheel active steer means that applies an auxiliary steering angle to a rear wheel and a four-wheel active steer control means that instructs both active steer means to apply the auxiliary steering angle to be a desired vehicular behavior characteristic so as to provide the four-wheel active steer system that applies no steering load onto a driver, and discloses that a steered state sensing means that senses a state steered by the driver, a response estimating means that estimates a response of at least either of the front-wheel and the rear-wheel active steer means and a response changing means that changes the other response according to the variation of the sensed steered state and the estimated one response are provided. A nonpatent literature 1 discloses that both yawing and a lateral motion can be optimized by actively steering both the front wheel and the rear wheel. For example, the nonpatent literature 1 discloses that all yawing (turning round a vehicle), the enhancement of a lateral acceleration response and the reduction of a skid of a vehicle body can be all realized by turning the front wheel by operating a steering at medium speed and also steering the rear wheel in the same direction at the same time. Especially, a nonpatent literature 8 discloses that immediately before steady turning, that is, after a yaw rate required for turning is acquired, the rear wheel is steered in same phase and stability is secured.
The four-wheel active steer systems disclosed in the patent literature 1 and the nonpatent literature 1 are respectively configured by respective actuators for actively steering the front wheel and the rear wheel and two electronic control units (ECU). The actuator for steering the front wheel is configured by parts such as a motor to be a driving source, a reduction mechanism, a turning angle sensor, a locking mechanism and a spiral cable for power supply. The actuator for steering the rear wheel is attached to a suspension member and steers the rear wheel via a suspension lower link after the rotation of a motor is converted to a translation motion in the deceleration mechanism.
(2) Direct Yaw-Moment Control (DYC)
Besides, in a patent literature 2, it is disclosed that the yaw moment is controlled by distributing driving force or braking force between right and left wheels of a vehicle, a feed-forward control device estimates a driving force distributed amount ΔT at which a yaw rate corresponding to a cornering behavior of the vehicle is acquired based upon engine torque, engine speed, vehicle speed, a steering angle and lateral acceleration so as to make a response and the precision of control compatible and feed-forward controls left and right hydraulic clutches CL, CR in a driving force distribution system. In the meantime, a feed-back control device calculates a deviation between a target yaw rate calculated based upon vehicle speed and lateral acceleration and an actual yaw rate sensed by a yaw rate sensor 10d and corrects the driving force distribution amount ΔT calculated in the driving force distribution system so as to make the deviation converge on zero. It is disclosed that even if the driving force distribution amount becomes excessive by feed-forward control and a trend of oversteer is caused in a vehicle, the trend of oversteer is eliminated by feed-back control and a behavior of the vehicle can be stabilized (refer to a nonpatent literature 2).
That is, a rear drive unit which is a DYC system and which is disclosed in the patent literature 2 and the nonpatent literature 2 is configured by parts such as a speed increasing gear unit including a high-low clutch, a planetary gear and an oil pump, a hypoid gear that converts a direction of drive, two right and left electromagnetic clutches and a planetary gear so as to make the distribution of torque between the right and left sides of a rear wheel free.
(3) G-Vectoring
A method of generating a load shift between a front wheel and a rear wheel by automatically accelerating or decelerating in coordination with a lateral motion by operating a steering and enhancing the maneuverability and the stability of a vehicle is also disclosed in a nonpatent literature 3.
An acceleration/deceleration command value for automatically accelerating or decelerating (target longitudinal acceleration Gxc) is acquired in the following mathematical expression 1.
                                              ⁢                  [                      Mathematical            ⁢                                                  ⁢            expression            ⁢                                                  ⁢            1                    ]                                                                              G          xc                =                                            -                              sgn                ⁡                                  (                                                            G                      y                                        ·                                                                  G                        .                                            y                                                        )                                                      ⁢                          Cxy                              1                +                Ts                                      ⁢                                                                          G                  .                                y                                                            +                      G            x_DC                                              (                  Mathematical          ⁢                                          ⁢          expression          ⁢                                          ⁢          1                )            Basically, the above-mentioned command value complies with a simple control rule that a value acquired by multiplying a lateral jerk Gy—dot by gain Cxy and applying a first-order lag is used for a longitudinal acceleration/deceleration command.
However, Gy: vehicular lateral acceleration, Gy_dot (|Ġy|): vehicular lateral jerk, Cxy: gain, T: first-order lag time constant, s: Laplace operator, Gx—DC: offset.
It is verified in a nonpatent literature 4 that hereby, a part of coordination control strategy of a lateral motion and a longitudinal motion of an expert driver can be simulated and the enhancement of the maneuverability and the stability of a vehicle can be realized. Gx—DC in this expression is a deceleration component (an offset) not linked with a lateral motion. The Gx—DC is a term required in a case of foreseen deceleration when a corner exists in front or when an interval speed command is issued. Besides, sgn (signum) is a term provided to acquire the above-mentioned operation both at a right corner and at a left corner. Concretely, operation that speed is decreased when steering is started and a turn is started, deceleration is stopped in steady cornering (because a lateral jerk is zero) and speed is accelerated when return in steering is started and in escape from a corner can be realized.
As in such control, resultant acceleration (expressed with G) of longitudinal acceleration and lateral acceleration is vectored to have curved transition in the elapse of time in a diagram having vehicular longitudinal acceleration on an axis of abscissas and having vehicular lateral acceleration on an axis of ordinates, the control is called G-Vectoring control.